This invention relates to printing inks and more particularly to infrared absorptive printing inks suitable for use in jet drop printers of the type disclosed in Brady et al, U.S. Pat. No. 3,805,273. In ink jet drop printing, a liquid ink is forced through a very small diameter orifice, normally in the range of 0.002 in. to 0.005 in., to form a stream of uniformly-sized, spaced spherical droplets. The droplets are formed at the rate of about 50,000 per second through each of 500 or more orifices in a manifold assembly. The stream of droplets is then directed, usually by electrical means which selectively charge the drops, onto the surface of a fast moving web, such as paper, where the stream forms printed characters in response an electronic character generator which controls the electrical charging and directing means.
As can be seen from the above description, the ink used in jet drop printing must possess a unique combination of properties. The ink must be electrically conductive, having a resistivity below about 1000 ohm-cm. and preferably below about 500 ohm-cm. It must have a workable viscosity in the range of about 1 to 10 centipose at 25.degree. C. and must be free of all but the smallest particulate matter to prevent plugging of the orifices. The ink should be stable over long periods of time, compatible with the materials used to make up the manifold assembly, free of living organisms, and, after printing, smear resistant, fast drying, and waterproof when dry.
Additionally, in systems where the printing must be capable of being read by automated optical character readers (OCRs) operating in the infrared range of about 7,000 to 11,000 angstroms, jet drop printing inks must also possess the property of being infrared absorptive while at the same time possess the capability of being readable by the human eye. Generally speaking, people are accustomed to reading printing which appears visually to be black. Thus, it is highly desirable that OCR readable jet drop printing inks be black as well as infrared absorptive. It is for this reason that although certain green dyes have known infrared absorbing characteristics, the prior art jet drop printing inks have generally utilized water soluble nigrosine black dyes.
Typical of such nigrosine dye ink compositions for use in jet drop printing are Zabiak, U.S. Pat. No. 3,705,043, and Edds et al, U.S. Pat. No. 3,870,528. Although nigrosine dyes are black to the eye and are infrared absorptive, they tend to agglomerate and are difficult to maintain in suspension for extended periods of time. As taught by Edds et al., there are solvents which, when added to the nigrosine compositions, appear to alleviate the agglomeration problem for jet drop printers which have only a relatively few orifices. However, such inks have not been fully satisfactory in printers of the type disclosed by Brady et al.
Accordingly, the need exists for an improved jet drop printing ink which is infrared absorbing at wave lengths utilized by optical character readers and appears visually black to the human eye.